Technical Field
The present invention relates to solar technology and more particularly to wafer bonded solar devices and fabrication methods to increase solar efficiency on multijunction solar cells.
Description of the Related Art
With growing concern about low-cost clean energy, solar power has again become a focal point for alternatives to fossil fuel energy production. Solar energy, while clean and sustainable, typically relies on expensive technologies for its implementation. These technologies include the incorporation of integrated circuits or integrated circuit technology into the fabrication of solar cells. The expense associated with current solar panels is a strong disincentive from moving in the direction of solar power.
Solar panels employ photovoltaic cells to generate current flow. When a photon hits silicon, the photon may be transmitted through the silicon, reflected off the surface, or absorbed by the silicon if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure. To achieve good carrier collection efficiency, multijunction cells have been developed. Multijunction cells include two or more cells stacked on top of each other. Any radiation transmitted through a top cell has a chance of being absorbed by a lower cell.
In theory, the more multijunction cells in a stack, the greater the overall efficiency should be. However, this is not the case with conventional devices. In practice, the efficiency of conventional tandem stacks progressively decreases from the theoretically estimated efficiency with each additional cell in the stack. For example, with a four cell stack the theoretical efficiency is about 50% and in practice the true efficiency is about 30%.
Tandem stacks suffer from many fabrication issues as well. As the stacks are grown (e.g., by epitaxial deposition methods), one on top of the other, lattice mismatches may occur between stack materials. This greatly affects material selection since the adjacent cells dictate which materials can be employed. Further, since band gap energies between adjacent cells need to be carefully selected, finding appropriate materials to form a tandem stack becomes extremely challenging.